-( This trophy can be overlooked, so be careful. )-
Act II (After the Wreck): After Robert is snatched by the Wind Monk, have the monk pull Robert up a few times from talking to him to drop the hook down to the ground below. Return to the forest floor and claim it. Now, talk to Gorf until you get the topic about the dry spot on its head. This is when you can use the hook on his head to scratch that pesky spot for it and get this trophy. Now, be aware that once you apply the liquid to his head, you won't be able to do this anymore.